Much effort has been made in recent years to develop low power color displays. Such efforts have generally employed LCD panels in one of three configurations.
In the first configuration, a plurality of differently colored LCD panels are stacked and illuminated with white light. As the light passes through the stacked layers, pixels in each panel act as controllable color filters, selectively coloring the light exiting the display. U.S. Pat. No. 3,703,329 is representative of such systems and shows a stack of three panels, variously dyed to individually produce the colors yellow, cyan and magenta. Together the panels cooperate, using subtractive color, to produce all eight primary colors. A related system is shown in U.S. Pat. No. 4,416,514. In this system, differently dyed polarizers (yellow, magenta and cyan) are interposed in a series of twisted nematic cells. By varying the voltage applied to each cell, the twist angle of the liquid crystal molecules changes, imparting a variable rotation to the light exiting the cell. The colored polarizers cooperate with this controllably twisted light to select desired colors.
While such stacked cell systems can provide a full color display, they typically have certain drawbacks. One is parallax, inherent in any stacked optical system. Another is poor brightness, due to absorption of light by the dye in dyed cell systems, and due to blockage of cross polarized light by polarizers in systems that rely on polarization rotation to differentiate colors.
The second approach uses only a single LCD panel, but uses it in conjunction with a mosaic color filter. The mosaic filter typically has a plurality of red, green and blue filter elements, each aligned with a pixel in the LCD panel. By controlling the transmissivity of pixels in the LCD panel, the display can pass light through selected areas of the color mosaic filter.
While the color mosaic technique addresses certain shortcomings of the stacked panel approach, it introduces certain problems of its own. One is that brightness is limited because, in the classical color mosaic approach, less than a third of the active pixel area transmits light for any given color. Another shortcoming of the classical approach is that pixel density must be increased by a factor of three to obtain the same resolution as the stacked cell approach. That is, to provide a color display with a horizontal resolution of 640 colored pixels, for example, the LCD panels must have 1920 pixels, 640 for each of the red, green and blue filter elements. This introduces fabrication problems that decrease yields and increase panel costs. Further, the finite width of the gap between pixels must remain, even though the pitch has decreased, so the actual pixel "aperture ratio" can be decreased dramatically. (Some small format thin film transistor (TFT) displays have a total open aperture area of only 30% of the total display surface due to row and column lines and transistor area, etc.)
The third approach is birefringence color. In such systems, the birefringent operating mode of certain materials is exploited to produce color, as opposed to reliance on colored dyes in guest-host type cells or reliance on rotation o light through known twist angles in twisted nematic cells.
Birefringent color systems typically take two forms: those relying on passive birefringent layers to impart a birefringent effect to a liquid crystal cell (as shown in U.S. Pat. No. 4,232,948), and those in which the liquid crystal material itself exhibits a birefringent effect (sometimes called "electrically controlled birefringence" or "tunable birefringence"). In the latter instance, the degree of birefringence is a function of the voltage applied to the liquid crystal material. By switching the applied voltage to different values, different colors can be produced. Color displays relying on this principle are shown in U.S. Pat. Nos. 3,785,721, 3,915,554 and 4,044,546.
During recent years, so called "supertwisted" or "highly twisted" nematic cells have become popular in many applications. Such cells are described, inter alia, in U.S. Pat. Nos. 4,697,884 and 4,634,229. Supertwisted nematic (STN) cells generally function in a birefringent mode. However, unlike earlier birefringent cells, STN cells exhibit a bistable behavior wherein they switch rapidly from a deselect (a.k.a. nonselect) state to a select state and back again as the excitation (RMS) voltage crosses a switching threshold. The select and deselect voltage regions can be made quite close to one another, such as 1.20 volts and 1.28 volts, permitting the cells to be multiplexed at high rates. FIG. 1 shows the transmission curve of a representative STN cell (with a particular polarizer orientation) as a function of applied voltage, illustrating the steepness of the switching function. Note that this curve shows the overall photopic "brightness" and does not reveal any coloration of the liquid crystal in the select and deselect states.
It is the multiplexibility of STN cells that makes them particularly desirable. This multiplexibility is achieved without active elements (i.e. drive transistors on each pixel, etc.) and without exotic alignment and liquid crystal operating modes (i.e. ferroelectric, phase-change, hysteresis, etc.). Thus, STN provides an inexpensive direct-multiplexed display type requiring only M+N drivers to operate a display comprised of M.times.N pixels.
There is an inverse-squared relationship between the number of display lines to be "addressed" and the difference between display "on" and "off" driving voltages (RMS average). As the number of display lines increases, the difference in driving voltage must decrease. To illustrate, a multiplex rate of 100:1 can be achieved with approximately a 10% difference in driving voltages, and a MUX rate of 240:1 can be achieved with approximately a 6% difference in driving voltages. Theoretically, arbitrarily high MUX rates can be achieved if the driving voltage difference is made small enough.
The main drawback to STN cells is the optical operating mode--birefringence. That is, the only way to distinguish pixels driven by the "on" voltage from those driven by the "off" voltage is the difference in birefringence between the two pixels. (As noted, for high information content displays, the difference in driving voltages is minute and decreases rapidly with an increase in the number of display lines that must be driven.) To distinguish the difference in pixel birefringence, polarizers are used--one to polarize the entering light to a known polarization, and one to select only one polarization of exiting light for examination. Depending on the state of the pixels, the light oriented to pass through the exit polarizer will be one of two colors. For best contrast, the polarizers are usually arranged so that these two colors are yellow and blue. (Actually, only one color can be selected by orientation of the polarizers--and this color can only be selected from a relatively small spectrum of colors. There is very little design freedom in varying the color in the second state--it is essentially a function of the first color.)
FIG. 2 shows the transmission characteristics of a representative yellow/blue mode STN cell (with associated polarizers) when the cell is in its select and deselect states. As can be seen, when the cell is "selected" (by applying an excitation voltage of 1.56 volts), the transmission spectrum has a maximum at 400 nanometers, a minimum at 600 nanometers, and an intermediate value at 500 nanometers. When the cell is "deselected" (by reducing the excitation voltage to 1.41 volts), the transmission spectrum includes a null at 400 nanometers, a maximum at 500 nanometers, and an intermediate value at 600 nanometers. Light exiting the cell/polarizer combination in the select state is thus principally blue, and light exiting in the deselect state is green plus yellow plus red, which appears as yellow to the human observer.
Unlike TN cells and cells operating in other modes, a birefringent STN cell cannot be operated in a black/white mode. The reason is that black requires all wavelengths of light to be linearly cross-polarized with the exit polarizer to effect complete light blockage, and true white requires all wavelengths of light to be linearly polarized parallel with the exit polarizer to effect complete light passage. The birefringent operating mode, by definition, prevents such results since different wavelengths of light are polarized differently during passage through the material, rendering a linear polarizer incapable of either blocking all or passing all wavelengths of the exiting light. Thus, STN cells are unavoidably colored. However, this drawback has been tolerated in order to achieve the high multiplexibility that STN provides.
In order to eliminate the birefringence color, some manufacturers have incorporated various compensation layers in display assemblies. One such compensation layer is a second birefringent cell of opposite twist than the first to counteract the wavelength dependence in the cell's behavior. While color effects are eliminated with this configuration, it is not possible to achieve both a high contrast ratio and a high multiplex rate.
Another type of compensation layer, sometimes used in conjunction with the above-mentioned blue/yellow mode STN LCDs, is a polarizer that has been dyed to pass cross-polarized light in the blue and red portions of the spectrum in order to make the yellow state of the LCD "whiter." This still yields a blue/white LCD, instead of the desired black/white. However, this color limitation is usually accepted in order to achieve the high multiplex ratio.
Further, while the birefringence of STN cells unavoidably produces colors, the colors so produced are generally considered too limited in range and too inferior in quality to be suitable for use in color displays. Far preferred are the rich colors that can be achieved with guest-host cells, or TN cells with dyed filters.
Another problem with prior art LCD displays is their relatively low resolution. The resolution of LCD panels is limited both by interconnection constraints and by the electrical properties of the liquid crystal material itself. Taking this latter limitation first, in any multiplexed LCD display, each cell must be electrically refreshed periodically, typically 30 or 60 times a second, to maintain its desired state. This is effected by repetitively scanning down the panel, refreshing each row in turn. The greater the resolution of a panel, the greater the number of rows that must be refreshed at this rate. Beyond a certain limit, the period allotted to refreshing each row becomes too short to refresh it effectively. Thus, a minimum refresh period limits the number of rows that can be refreshed at the requisite rate. This number is about 250-300 rows with current liquid crystal materials.
In the prior art, displays with twice this number of rows have been achieved by duplicating the refresh circuitry so that half the rows of the panel are refreshed by one circuit and half are refreshed by the other. Thus, at any instant, two rows are being refreshed--one by one circuit and one by the other. However, this technique still only permits 500 or so rows of resolution. Truly high resolution applications demand substantially more rows.
The obstacle to refreshing more than 500 rows is the interconnection limitation. The refresh circuitry must connect to each column of pixels on the display. There may be 640 or more such columns. By partitioning the display into top and bottom portions, the two requisite 640 wire connections can be made--one along the top of the display and one along the bottom. However, this partitioning approach cannot be extended to a three- or more way division because there is no way to make the requisite interconnect to intermediate portions of the display.
The interconnect limitation is generally accepted to be an absolute bar to arbitrarily-high resolution LCD displays, as noted in "Scanning Limitations of Liquid Crystal Displays" by P.M. Alt et al, IEEE Trans. Electron Devices, Vol. ED-21, pp. 146-155 (1974); and "Ultimate Limits for Matrix Addressing of RMS-Responding L.C.D.'s" by J. Nehring et al, IEEE Trans. Electron. Devices, Vol. ED-26, p. 795-802 (1979).